1. Field of the Invention
The present invention relates in general to the field of laser resonators, and more particularly to a new injection-controlled laser resonator which incorporates self-filtering, self-imaging, and adjoint-coupled injection features.
2. Background Art
A conventional laser system 1 is illustrated in FIG. 1 typically includes a gain medium 2 such as plasma, solid state crystals or similar gain media used for signal amplification, and a cavity resonator which comprises two reflective surfaces or cavity mirrors 3 and 4 that are oppositely disposed relatively to the gain medium 2, such that part of the laser beam is trapped in the resonance cavity and is re-amplified by the gain medium. As known in the art, pump means are provided for pumping the gain medium to a pumped state. For a copper vapor lasers, the pump means includes means for promoting the vapor state of copper lasing material. A typical laser system includes the use of a highly reflective concave cavity mirror 3 and a reflective convex cavity mirror 4, with a scraper mirror 5 in between for the laser output. When the lasing atoms are excited, they emit photons in the gain medium.
The photons are initially emitted in all directions, and some of which impinge upon the concave cavity mirror 3, which causes these photons to be reflected back through the gain medium 2 where they are re-amplified. The laser beam is then intercepted by the scraper mirror 5 as an output beam. Part of the laser beam passes through a central feedback aperture 6 in the scraper mirror 5, for impinging on the convex cavity mirror 4. The latter mirror 4 reflects the impinging laser beam back divergently through the feedback aperture 6 and the gain medium 2 where it is re-amplified before impinging upon the highly reflective concave mirror 3.
The foregoing resonance cycle is then repeated, and the lasing operation proceeds. The laser beam which is trapped between the two cavity mirrors 3 and 4, within the resonance cavity, forms resonator or laser modes. Numerous designs of laser resonators have already been developed, and can be generally classified as in two categories: stable and unstable resonators. The resonator is said to be stable if it satisfies the following equation: EQU 0&lt;(1-L/R1)*(1-L/R2)&lt;1,
where L is the distance between the two cavity mirrors; R1 is the radius of one of the cavity mirrors; and R2 is the radius of the other cavity mirror. If a resonator does not satisfy the foregoing equation, it is said to be unstable.
A stable resonator has a lower cavity loss than an unstable resonator, but requires a longer time for its cavity modes to be developed. A stable resonator is mostly used in low gain, long pulse or continuous wave laser systems. An unstable resonator has a higher cavity loss than the stable resonator, and does not require as much time for its cavity modes to be developed. An unstable resonator is mostly used in high gain, short pulse laser systems.
Another distinction between the stable and the unstable resonators is that a stable resonator has a magnification factor (M) of one, while the magnification factor M of an unstable resonator is defined as the ratio of the mirror radii, as follows: EQU M=R1/R2.
Thus, a laser beam that is transmitted through the resonance cavity will be magnified by a factor M after one cavity round trip. As used herein, a cavity round trip means the main and the return trajectories traveled by the laser beam between the two cavity mirrors. The higher the magnification factor M, the faster the cavity modes can be developed.
Even though laser modes can be developed relatively quickly using high magnification (i.e., M greater than 10) unstable resonators, the time required for such development may still be too long for some short pulse laser systems. For example, the typical time required for the development of oscillator modes in high gain lasers, such as copper lasers and excimer lasers, is in the order of a few tens of nanoseconds (i.e., 30 nsec.) using high magnification unstable resonators, which is in the order of the entire laser pulse duration (i.e., 60 nsec.). Therefore, the front part of the laser pulse, which contributes a large amount of output energy, is not a well developed resonator mode, and is therefore highly divergent (i.e., poor beam quality).
In order to achieve a low divergence output beam for obtaining a good beam quality for the entire duration of the laser pulse, injection control techniques are usually applied. An injection signal is generated by means of a lower power laser, which is referred to as the master oscillator (MO), and the output laser pulse will be composed of two parts, a leading part and a trailing part. When an unstable resonator is used as the master oscillator, the trailing part of the output laser has a developed resonator mode (i.e., low beam divergence) and can be used as a seeding beam for injection purposes. The main or high power laser which uses either a stable or unstable resonator, is then injected with the seeding beam, which has a relatively good beam quality. As a result, the high power laser pulse from the injection seeded oscillator (ISO), which is initially seeded with a low divergence laser pulse, can maintain the low divergence mode for the entire duration of the laser pulse. This approach constitutes a substantial improvement over a free-running oscillator which can only develop a low-divergence laser beam for the trailing part of the output laser pulse.
The most commonly used injection scheme is the phase matched injection which is achieved by matching the injected wavefront to the ISO divergent wavefront. FIG. 2 illustrates a conventional injection-controlled laser resonator 10, which is similar to the laser system 1 of FIG. 1, and which further includes an injection mirror 14 which reflects an incoming injection beam 16 toward the mirror 3, through the feedback aperture 6 of the scraper mirror 5.
However, it was found that the leading part of the ISO output consists of a re-amplified MO injection beam with its direction and collimation controlled by the injection beam. As soon as the seeding beam is extinguished, the ISO cavity modes start to dominate, and the laser beam output becomes ISO mode dominated, with its direction and collimation controlled by the ISO cavity. This characteristic, accompanied by small injection misalignment, results in undesirable multiple far-field (or focused) spots. In most laser applications, a single far-field spot is highly desirable.
Another concern is that the quality of the seeding beam is usually not diffraction limited, i.e., a laser beam quality can not be better than the diffraction limited, because of the imperfection of the master oscillator (MO) cavity design. This beam quality degradation will be carried over to the injection seeded oscillator (ISO) beam quality.
Yet another main concern associated with the use of an injection seeded oscillator (ISO) is the edge waves generated by the diffraction of the feedback beam at the feedback aperture. These edge waves induce small-scale phase aberrations and high order modes, which further enlarge the laser beam divergence.
It would therefore be highly desirable to have a new injection-controlled laser resonator which eliminates the dual far-field spots problem, improves the injection beam quality, and minimizes the edge waves effect, with diffraction limited beam quality.